A hollow-shaft rotor cooling system for automotive traction motors

Abstract

Automotive traction motors with demanding power density requirements, are exposed to higher temperature operations, which can have a detrimental effect on the efficiency, reliability and life expectancy of these motors. In order to minimize this level of damaging, a highly effective cooling and ventilating system has to be incorporated into the optimal design of such motors. It can ensure sufficient heat removal from the machine inside. On top of this, a better cooling performance of the rotor will result in higher power density, better field weakening capability and reduced costs. This thesis addresses a hollow-shaft rotor cooling system for automotive traction motors. In such cooling system, the coolant is forced to pass through a hollow-shaft in order to cool the motor. However, in such case, the heat transfer can be complex due to the secondary flow which will occur as a result of the shaft rotation. As a result, the convective heat transfer coefficient (HTC) correlation of a stationary case is invalid for a hollow-shaft rotor cooling system. The aim of this thesis is to investigate the effects of the rotational velocities, coolant flow rates and coolant temperature, on the thermal performance of this cooling method. A simplified numerical model based on computational fluid dynamics (CFD) methods was developed to provide a qualitative understanding of the mechanism of convective heat transfer. Then a finite element model (FEM) was constructed to represent the heat transfer of the test rig by considering heat dissipation of conduction and convection. Experiments were then carried out to validate the accuracy of CFD models with the assistance of an analytical lumped-parameter thermal network (LPTN) approach. As a result of such analysis, two new dimensionless correlations of the Nusselt number with the Reynolds number are derived for turbulent and laminar flow, respectively. These correlations can be applied in different geometrical contexts with various axial and rotation flow rates. Finally, an accurate evaluation of the flow resistance in a rotating shaft has been presented. The rotational friction loss factor in such a hollow-shaft is studied, where the effect of the shaft velocity as well as the coolant flow rate have been accounted for.